Dr Simpson said the research showed that almost 22 per cent of breast tumours had a genetic signature reflecting a BRCA deficiency, even though the patient had not always inherited a mutation in one of the two genes.

Dr Peter Simpson

“BRCA genes are DNA repair genes, so if there is a fault or mutation in one of these genes there is more susceptibility for a patient to develop a disease such a breast cancer,” Dr Simpson said.

“Between one and five per cent of breast cancer cases are due to inherited BRCA1 or BRCA2 mutations.”

The researchers developed a computer program to identify tumours with a BRCA deficiency, based on the appearance of the genome.

Professor Lakhani said the tool could be incredibly useful from a treatment point of view, because if a patient had this type of DNA repair deficiency, it opened up the possibility that the tumour would be responsive to a particular type of drug called PARP inhibitors.

“These PARP inhibitor drugs are currently in clinical trials where most of the focus is on patients with a genetic deficiency in BRCA1/BRCA2,” Professor Lakhani said.

“This study has identified a much larger patient group who could benefit.

“It could be an effective way of picking those patients where particular treatments are going to work, and that’s a big step forward in cancer research.”